Electric water heaters are used to heat and store a quantity of water in a storage tank for subsequent on-demand delivery to plumbing fixtures such as sinks, bathtubs, showers, and appliances in residential and commercial buildings. Electric water heaters typically utilize one or more electric resistance heating elements to supply heat to the tank-stored water under the control of a mechanical or electrical thermostat device that monitors the temperature of the stored water.
Storage-type electric water heaters typically include one or more heating elements to which electric current may be applied to thereby generate resistive heating. Both elements, assuming there are two, extend into the tank volume so that water within the tank receives heat directly from the elements. A control system controls the connection of electric current to the heating elements responsively to a comparison of water temperature to predetermined temperature set points. For example, the water heater may include a temperature sensor as a thermistor or bimetallic switch disposed on the outer surface of the water tank proximate a respective heating element so that the temperature sensor is responsive to temperature of water in the tank near the heating element. In the case of a bimetallic switch, the switch is configured to open at a predetermined high temperature (i.e. the high set point temperature) and close at a predetermined low temperature (i.e. the low set point temperature). In turn, the bimetallic switch controls the operation of a switch in the electric current path between line current and the heating element. Thus, if the bimetallic switch detects that water in the tank is at or below the lower set point, the bimetallic switch closes, thereby closing the switch in the electric current path and providing electric current to the heating element. This causes the heating element to generate resistive heat, thereby increasing temperature of water in the tank. The bimetallic switch continues to sense the tank water's temperature as that temperature increases. When the switch detects that the temperature has reached the high set point, the switch opens, thereby opening the circuit switch and disconnecting the electric current source from the heating element and, therefore, deactivating the heating element. The bimetallic switch remains open as the tank water cools but closes again when the now-cooler water reaches the low set point, and the cycle repeats. A similar process occurs through operation of the bimetallic switch at the lower heating element. In water heaters using thermistors, the respective thermistors at the two heating elements output signals to a water heater controller that compares the temperatures represented by the signals to high and low set points stored in memory and controls relays that, in turn, open and close switches in the electric current paths between line current and the heating elements. The processor controls activation of the electric current switches responsively to the temperature signals from the thermistors to thereby activate the heating elements when the cooling tank water reaches the low set point and deactivate the heating elements when the now-heating water reaches the high set point, similar to the cycles executed by the bimetallic switches.
Under existing regulations, electric water heaters having a rated storage capacity of greater than 55 gallons are required to have an energy factor of 2.057 or greater. However, electric utilities that operate demand response programs may rely on electric water heaters in order to reduce/shift power usage during peak demand periods. As such, there is a need for the usage of electric water heaters with large storage capacities, even where achieving the required energy factor may not be feasible. With this in mind, Congress has enacted the Energy Efficiency Improvement Act of 2015, which allows the manufacture and sale of electric water heaters having rated storage tank capacities of greater than 75 gallons, as long as the electric water heaters include an activation lock that can only be activated by the utility. The activation lock is to be provided at the point of manufacture and, when in the locked position, disables a number of the water heater's electric resistance heating elements so that the required energy factor is achieved. The manufacturer of the electric water heaters provides an activation key for unlocking the activation lock, thereby allowing the flow of current to the previously disabled electric resistance heating elements, only to the utility conducting the demand response program in which the water heater is to be utilized. Only after the electric water heater is enrolled in the corresponding demand response program does the utility unlock the activation lock, thereby allowing current to flow to the corresponding electric resistance heating elements.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles of the invention.